1. Field of the Invention
The present invention relates to a technique of correcting a frequency of an oscillator, and particularly to a technique of automatically correcting a frequency of a reference oscillator in a terminal of a mobile communication system using CDMA method or the like.
2. Description of the Related Art
In recent mobile communication, a high frequency band from 900 MHz to several GHz has been used. Therefore, even though a TCXO (temperature compensated crystal oscillator) having high precision such as a frequency error of almost 3.0 ppm is used as a reference oscillator, a frequency error of 3 kHz or above can occur. Therefore, a frequency deviation caused by the frequency error of the reference oscillator occurs between a transmitter and a receiver. When the frequency deviation between the transmitter and the receiver is enlarged, a signal received in the receiver cannot be correctly de-modulated.
When precision of TCXO used for the reference oscillator is heightened, the frequency deviation is lowered. However, the cost of a terminal is inevitably increased. Therefore, a technique has been conventionally proposed that the frequency deviation is lowered by adjusting a frequency oscillated by TCXO (For example, refer to Japanese Patent Application Publication No. H6-326740).
Referring to FIG. 1, a conventional mobile terminal has antennas 501 and 514, receiver 502, de-modulator 503, decoder 504, RX-synthesizer 505, TCXO 506, D/A converter 507, controller 508, frequency error detector 509, coder 510, modulator 511, TX-synthesizer 512 and transmitter 513.
Receiver 502 converts a radio frequency signal transmitted from a base station (not shown) and received in antenna 501 to an intermediate frequency signal and transmits the intermediate frequency signal to de-modulator 503. Because receiver 502 uses a frequency signal obtained in RX-synthesizer 505 which uses a reference frequency of TCXO 506, a frequency of the intermediate frequency signal produced in receiver 502 includes an error based on an oscillation frequency error of TCXO 506.
De-modulator 503 de-modulates the intermediate frequency signal transmitted from receiver 502 and transmits obtained received base band signals (RX-I, RX-Q) to decoder 504 and frequency error detector 509.
Decoder 504 decodes the received base band signals and transmits obtained received DATA to a circuit (not shown) of a subsequent stage in synchronization with clock.
Frequency error detector 509 detects a frequency error Δf from the received base band signals, for example, by measuring a difference in phase between slots, and transmits the frequency error Δf to controller 508.
Controller 508 generates a frequency error compensating signal (hereinafter, simply named “control signal”) DVc lowering the frequency error  Δf to a predetermined value or below and transmits the control signal DVc to D/A converter 507.
D/A converter 507 performs digital-to-analog conversion for the digital control signal DVc and gives such an obtained control voltage Vc to TCXO 506.
In TCXO 506, an oscillation frequency of a crystal oscillator (not shown) is controlled in voltage by using the control voltage Vc. TCXO 506 gives an oscillation frequency obtained by the control to RX-synthesizer 505 and TX-synthesizer 512 as a reference frequency.
By performing the control of controller 508 for TCXO 506 so as to lower the frequency error Δf detected in frequency error detector 509 to the predetermined value or below, the reference frequency in TCXO 506 is synchronized with a received signal and is stabilized.
RX-synthesizer 505 generates a frequency signal of a desired frequency from the reference frequency and transmits the frequency signal to receiver 502 and de-modulator 503.
TX-synthesizer 512 generates a frequency signal of a desired frequency from the reference frequency and transmits the frequency signal to transmitter 513 and modulator 511.
Coder 510 receives Transmit DATA synchronized with clock from a circuit (not shown) at a preceding stage, codes the Transmit DATA and transmits the coded Transmit DATA to modulator 511 as transmit base band signals (TX-I, TX-Q).
Modulator 511 modulates the intermediate frequency signal with the transmit base band signals and then transmits it to transmitter 513.
Transmitter 513 converts the intermediate frequency signal transmitted from modulator 511 to a radio frequency signal and transmits the radio frequency signal to a base station (not shown) through antenna 514.
Referring to FIG. 2, controller 508 has register 601, adder 602 and multiplier 603.
Multiplier 603 multiplies a signal of the frequency error Δf by a coefficient “a” and transmits thus obtained signal to adder 602. Adder 602 adds an output of register 601 and an output of multiplier 603 with a predetermined adding polarity and transmits an obtained value to register 601. In the example of FIG. 2, adder 602 subtracts the output of multiplier 603 from the output of register 601.
Register 601 temporarily stores and delays an output of adder 602 and transmits the output to D/A converter 507 and adder 602. A summing circuit is composed of register 601 and adder 602, and outputs of multiplier 603 are summed in the summing circuit. A control signal DVc indicates a result of the summed outputs.
Referring to FIG. 3, the relation between the control voltage Vc given to TCXO 506 and an amount of change in the reference frequency based on the control voltage Vc can be expressed by a substantially straight line. Accordingly, when the adding polarity of adder 602 is correctly selected so as to lower the frequency error, the reference frequency in TCXO 506 can be converged so as to be synchronized with the received signal transmitted from a base station (not shown).
The configuration described above is a typical example of a frequency correction apparatus in a conventional mobile terminal. As described above, in the conventional mobile terminal, a frequency error is detected according to some method, the frequency error is given to a reference oscillator, and a reference frequency is corrected.
However, there exist the following problems in the above-described prior art.
In CDMA mobile communication represented by IS95 of the United States, a base station superposes frequency-spread signals of a plurality of channels on the same frequency. Further, a plurality of base stations use the same frequency, and each base station transmits signals of a plurality of channels of different spreading codes at the same frequency.
Therefore, electric waves are received in the mobile terminal with a plurality of channels transmitted from a plurality of base stations existing by mixture in the same frequency. In CDMA, this considerably differs from an analog method and a digital method of TDMA. Each of channels existing by mixture in the same frequency is distinguished by using a spreading code used for the corresponding frequency spreading.
Only when the mobile terminal performs complicated types of processing such as base station search, synchronization, frequency despreading and the like, the mobile terminal can extract a signal addressed to itself from signals of a plurality of channels existing by mixture in the same frequency. Further, only when the mobile terminal extracts a signal addressed to itself, the mobile terminal can detect an error (frequency deviation) between a frequency of the signal addressed to itself and a reference frequency of itself. Accordingly, in the mobile terminal of the CDMA mobile communication, unless the complicated processing are correctly performed in de-modulator 503, no frequency error can be detected in frequency error detector 509.
Further, to perform normally the complicated processing such as base station search, synchronization, frequency despreading and the like, an error of the reference frequency is required to be substantially small. To perform normally the complicated processing, a severe condition is given to the reference frequency in TCXO 506 that a deviation between the reference frequency and a frequency of a signal transmitted from a base station is, for example, within ±3.0 ppm.
When the reference frequency in TCXO 506 does not satisfy this condition, frequency error detector 509 cannot detect the frequency error, and the mobile terminal cannot correct the reference frequency.
Therefore, in order to start the correction of the reference frequency in the mobile terminal, even in condition that the correction to be performed by giving the frequency error detected in frequency error detector 509 to TCXO 506 is not performed, TCXO 506 is required to generate a reference frequency having precision which satisfies a severe condition such as with ±3.0 ppm.
As causes of changing a reference frequency oscillated by TCXO 506 and generating error, there are temperature and time changes.
As to the change of the reference frequency due to change of temperature, for example, even though an oscillator with the highest performance currently available is used as TCXO 506, there is a probability that a frequency change such as within ±2.0 ppm in the worst case occurs in a temperature range of −35° C. to +85° C. in which the mobile terminal should be guaranteed to be able to be operated.
In FIG. 4, theoretical values of the change of the oscillation frequency due to change of temperature are shown by a solid line on condition that no secular change exists. In the example of FIG. 4, in case of no secular change, the oscillation frequency satisfies the condition that an amount of change is within ±3.0 ppm in a range of −35° C. to +85° C. centering around 25° C.
In FIG. 5, a secular change of an oscillation frequency of a crystal oscillator is exemplarily shown in which the oscillation frequency is lowered by 0.5 ppm per year. Further, in order to show only influence of the secular change, influence of temperature is removed.
As shown in FIG. 5, even though the oscillation frequency of the crystal oscillator in TCXO 506 is correctly adjusted just before being shipped from the factory shipping time, when the crystal oscillator is used for a long term, the oscillation frequency changes due to a chemical change of the crystal oscillator. In the example of FIG. 5, the oscillation frequency becomes lower by 0.5 ppm in one year, 2.5 ppm in five years and 5.0 ppm in ten years.
As shown in FIG. 6, in the oscillation frequency of the crystal oscillator, there is a short term change due to a temperature change superposing on a secular change gradually changing for a long term. As understood from FIG. 4, for example, the oscillation frequency changes due to influence of a temperature difference between night and daytime. When the crystal oscillator is placed out of doors in a cold area in winter, the crystal oscillator cooled to −10° C. or below is quite possible. When the crystal oscillator is placed in an automobile during daytime in midsummer, it is quite possible the crystal oscillator can become heated to 60° C. or above.
Further, the temperature of a mobile terminal is changed in dependence on operation conditions. When the mobile terminal is set in a power-off state, because there is no heat generated due to electric power consumption, the mobile terminal is sufficiently cooled at a time just after power turn-on and is set at a low temperature. However, when time has sufficiently elapsed after power turn-on, even though the mobile terminal is set in a standby state, temperature of the mobile terminal is elevated by heat generated due to electric power supply. Further, because a transmission amplifier is heated due to transmission of electric waves in a talking state, the mobile terminal is heated to a highest temperature. Furthermore, at a changing time of each state described above, there exists an unstable period in which temperature of the mobile terminal is transitionally changed. As described above, the oscillation frequency of the crystal oscillator of TCXO 506 frequently changes in a short period due to the temperature change.
In contrast, the oscillation frequency of the crystal oscillator is changed very slowly due to a secular change. For example, in a period of almost one month, a secular change of the oscillation frequency is very small and within range of a negligible error. However, like 2.5 ppm in five years in FIG. 5, when a secular change in several years is observed, the oscillation frequency has undoubtedly changed due to the secular change.
In a short period, an amount of change in the oscillation frequency is within range of about ±2.0 ppm due to the temperature change, and there is no problem in operation. However, in a long term, a secular change is added to the change of the oscillation frequency. Therefore, when four or five years have elapsed, an amount of change can be out of ±3.0 ppm, denoting an allowable range. When the time has further elapsed, the probability of the amount of change placed out of ±3.0 ppm is further heightened.
As described above, when the secular change of the crystal oscillator is left as it is, the reference frequency of TCXO 506 changes little by little, and the correction to be performed by giving the frequency error detected in frequency error detector 509 to TCXO 506 eventually cannot be performed. Therefore, when several years have elapsed after the shipping has begun, there is a probability that operational failure will occurs in a large number of mobile terminals.
As one of the countermeasures, an initial value of the control signal DVc for TCXO 506 at a time just after power turn-on is, for example, set to a convergent value previously obtained at a time before power turn-off. However, it is uncertain that temperature at the current time is almost the same as that at the previous time, and there is a probability that temperature at the current time considerably differs from that at the previous time. Accordingly, in case of this counter measure, in dependence on the temperature difference, there is a probability that conditions will become worse than those in case of no adoption of this countermeasure, is not adopted.